Electrical instruments having rotary operating members



2,815,422 ELsc RIcAL INSTRUMENTS HAVING ROTARY OPERATING MEMBERS Filed Ma 14, 1956 h T. LOCK Deg. 3,1957

2 Sheets-Sheet 1 A ttorn e y MEMBERS T. LOCK Dec. 3, 1957 ELECTRICAL INSTRUMENTS HAVING ROTARY OPERATING 2 Sheets-Sheet 2 Filed May 14, I956 inventor 75., M 3 2 4 40- A tlarn ey United States Patent ELECTRICAL INSTRUMENTS HAVING ROTARY OPERATING MEMBERS Tom Lock, Kingsthorpe, England, assignor to Painton & Company Limited, Kingsthorpe, England, a British company Application May 14, 1956, Serial No. 584,820 Claims priority, application Great Britain May 12, 1955 8 Claims. (Cl. 201-56) This invention concerns improvements in or relating to electrical instruments having a rotary electrical member driven by an operating spindle. By the term rotary electrical member, we mean a part of the instrument which part is rotatable relatively to another part or other parts of the instrument to produce a variation of some electrical property of the instrument. More especially, the invention is concerned with the provision in an electrical instrument, particularly one of miniature size, e. g. having, for example, an external diameter in the region of 1 /2" and an overall length including the operating spindle of 2%", of an operating assembly so constructed that the rotary electrical member cannot be damaged by mishandling of the operating spindle. The invention is particularly useful in its application to miniature variable resistances or potentiometers, although it can also be applied to other electrical instruments, such as variable electric condensers, inductances, rotary switches and the like, having an electrical member adapted to be rotated through the medium of a rotary operating spindle.

The operating assembly of an instrument according to this invention provides an indirect drive between the op erating spindle and rotary electrical member and stop means in the said indirect drive so that rough manual actuation of the operating spindle does not damage the rotary electrical member as frequently occurs in the case of known instruments in which the operating spindle drives the rotary electrical member directly and stop means are provided on the rotary electrical member.

According to one aspect of this invention there is provided an electrical instrument comprising: a body; a mounting bush secured to said body for the purpose of mounting the instrument to an appropriate support and having a screw-threaded part; an electrical member which is carried by said body for rotation relatively thereto and the rotation of which causes variations of an electrical property of the instrument; an operating spindle mounted in the instrument immovable axially of the said body, but rotatably relatively thereto; such operating spindle having a diametrical bifurcation in one end thereof; an intermediate driving element drivingly connected to the said electrical member and positioned coaxially of the operating spindle at the end thereof having said bifurcation; a key ing boss furnished diametrically across that end of the driving element which is nearer to the operating spindle such keying boss engaging as a free sliding fit within the bifurcation of the latter and having a screw-threaded part engaged with the said screw-threaded part of the mounting bush so that rotation of the operating spindle causes rotation and relative axial movement of the driving element and stop means to limit the axial movement of the driving element.

Said stop means advantageously comprises one or more stop elements movable axially with the keying boss and engageable against one or more axially immovable parts of the instrument at opposite ends of the travel of the boss. Thus the keying boss may have a transverse pin extending into a longitudinal slot or slots provided in the 2,815,422 Patented Dec. 3, 1957 flank or flanks of the bifurcation of the operating spindle, the length of the said slot or slots being so determined as to permit to the operating spindle only that degree of rotation, e. g. three, four or five revolutions, that is required, whereby the rotary electrical member cannot be traversed beyond its end positions. If desired, the said pin may have its outer ends screw-threaded to engage with the internal thread of said relatively stationary part of the instrument.

In applying this invention to helical wire wound resistances or potentiometers (hereinafter referred to as potentiometers) the said rotary electrical member, which in this case is a brush for engaging the helically coiled resistance element of the potentiometer, may conveniently be carried by a brush carrier located within the resistance element and rotatable by said driving element around, and simultaneously traversable along, the axis of the resistance element in such a manner as to traverse the said brush along the helical path of the said resistance element.

Desirably, the brush carrier may be in the form of a nut screwed on to a lead screw mounted coaxially within the resistance element, this lead screw being non-rotatable relatively to the resistance element and the said nut being rotatable on the lead screw of which the thread is of the same pitch as the helix defined by the resistance element. Conveniently the screw has a multi-start, e. g. a threestart thread.

Advantageously the driving element may be of forked form, the arms of the fork embracing radial lugs on the brush carrier so that the latter is free to move axially of the resistance element and relatively to said driving element whilst simultaneously being rotated about the said lead screw.

Conveniently the potentiometer may comprise a hollow body or housing (hereinafter called body) having the helically coiled resistance element insulatedly positioned therein and, although the electrical return from the resistance element may be made through the brush carrier, driving member and operating spindle, desirably a coiled return or collector element (hereinafter called a collector element) is positioned within the said body concentric with the resistance element, but axially ofiset therefrom, and the brush carrier carries two brushes electrically interconnected, one brush engaging the resistance element and the other brush engaging the collector element.

The said hollow body may conveniently comprise a moulded electrically insulated shell of generally cylindrically internal form, the interior of the body having two helical grooves, one to receive the helically coiled resistance element and the other to receive the helically coiled collector element, both grooves being of the same pitch and one (preferably that for the collector element) being of smaller radius than the other.

The said body is preferably of a split, advantageously two-piece, form so that it can be readily assembled about the parts it is to contain and, if desired, an outer casing or sheath may be provided to contain and protect and/ or seal the said body from the atmosphere.

The said resistance element conveniently comprises a helically coiled former upon which a resistance wire is helically wound and if the former is, as conveniently is the case, made of an electrically conducting material such as wire, then the resistance wire wound helically around the former is electrically insulated from the same, this insulation conveniently being achieved by means of a thermosetting resin which also can be made to serve the purpose of positioning and fixing the resistance wire on the former.

The collector element may conveniently be in the form of a helically wound spring wire, such as a nickel silver wire.

In order that this invention may more readily be understood reference will now be made by way of example to the accompanying drawings in which:

Figure 1 is a longitudinal section through a potentiometer including an operating assembly according to this invention, this operating assembly being shown mainly in elevation;

Figure 2 is a cross-sectional view on the line II-ll of Figure 1;

Figure 3 is a part sectional view on the line llliil of Figure 2; and

Figure 4 is a fragmentary longitudinal section through a potentiometer having a modified operating assembly.

Referring to Figures 1 to 3, the invention is shown a plied to a potentiometer, generally indicated by the reference 1, having a resistance of the order of 100,000 ohms, this potentiometer broadly comprising a linear wound helical resistance element 2 giving three complete turns, i. e. a minimum of 1080 electrical and mechanical rotation; a helical wire collector element 3; a brush unit 4 including a brush carrier 5 and a double ended pick-up brush 6 mounted on the carrier 5 and rotatable on a fixed lead screw 7 by means of an operating assembly 8; and a mounting bush 9 which, together with the aforementioned parts, is assembled in a two-piece moulded body 10.

The potentiometer body 10 is externally generally cylindrical and is diametrically divided into two halves and furnished at its external periphery with transverse lugs 11 and bores 12 to receive bolts 13 for clamping the two halves of the body rigidly together, the bolts preferably being arranged, as shown, at right angles to the meeting faces of the two halves of the body of the potentiometer.

The body 10 is formed as a two piece moulding of suitable electrically insulating material, such as, for example, phenol formaldehyde synthetic resin, the body being closed at one end and having a central circular opening 14 at the other end in which fits the mounting bush 9, the latter having a cylindrical external screw threaded reduced extension 15 projecting axially outwardly from the operating end of the body it).

Internally, the body 10 is furnished coaxially with a three turn helical groove 16 for receiving and positioning the resistance element 2, the formation of the groove 16 providing a lelical web 17 between the successive convolutions of the groove 16. In the inner periphery of the helical web 1'7 is formed a three-turn helical groove 13, preferably V-shaped as shown, in which is mounted the helical collector element 3.

The pick-up brush 6, which is the rotatable electrical member as herein defined of the potentiometer, comprises a shaped length of spring wire of high electrical conductivity, such as a palladium alloy wire, fixed in the brush carrier 5 which is of insulating, preferably moulded material, the wire constituting the brush having a part 19 for engagement with the resistance element 2 and a similar part 20 for engagement with the collector element 3.

The brush carrier 5 is preferably moulded from nylon and is in the form of a nut screwed on to the lead screw 7 which is arranged co-axially of the body 10 of the potentiometer, the thread of the brush carrier conveniently being moulded thereinto and matching the thread of the lead screw but so formed as to provide sufficient friction between the lead screw and the brush carrier to prevent any accidental variation of the relative positions of the brush carrier 5 and lead screw 7.

The lead screw 7 has a three-start thread of a pitch which matches the pitch of the convolutions of the resistance and collector elements and at one end. the screw 7 has a hexagonal head 21 which is freely, but non-rotatably, received into a hexagonal recess 22 provided in the closed end of the body it) of the potentiometer, this recess being moulded half in each of the two mating parts of the body.

It will be appreciated that, as the brush carrier 5 rotates on the lead screw 7, the parts 19 and 20 of the brush 6 describe helical paths and respectively wipe over the resistance element 2 and collector element 3.

The operating assembly 8 for rotating the brush carrier 5 comprises an operating spindle 23 which drives an intermediate driving element 24, the latter including a fork 25 of generally hollow cylindrical slotted form and arranged coaxially around the lead screw 7, the arms 26 of the fork 25 embracing radial lugs or wings 27 on the brush carrier 5.

The fork 25 is made of fibre, of a moulded or laminated plastic, or of another suitable electrically insulating material of good mechanical properties and is secured, e. g. by screws 28 at the operating end of the body iii, to the inner end of a fork extension 29 in the form of a cylindrical metal, e. g. stainless steel, block machined away symmetrically on both sides of its axis to provide a diametrically arranged keying boss 30 having a pair of oppositely arranged parallel flats arranged upon parallel chords of the original cylinder and provided on its opposite curvilinear outer peripheral portions with a screw thread adapted to co-operate with an internal screw thread 31 provided in an enlarged inner end portion of the mounting bush 9. The operation of the keying boss 9 is such that, when the fork Z5 and its extension 29 are rotated about their own axes relatively to the potentiometer body 10, axial adjustment of the fork 25 and extension 29 relatively to the mounting bush 9 and the body it takes place in one or the other axial direction according to the direction of relative rotation of the fork 25 with respect to the body 110.

Rotation of the fork 25 and extension 29 is affected by the operating spindle 23 which is rotatably mounted in the bore of the reduced extension 15 of the mounting bush 9, the spindle 23 being restrained against axial movement in the bush 9 by being provided at its forward part immediately in front of the extension 15 with a groove 32 in which is fitted a circlip 33 and at its inner end with an enlarged head 34.

The outer end of the operating spindle 23 is adapted to receive a knob or other operating member by which the spindle can be rotated whilst the inner head 34 of this spindle is diametrically bifurcated to receive the diametrically arranged fiat-provided and screw threaded keying boss 30 of the fork extension 29 in such a way that, when the operating spindle 23 is rotated, the fork 25 is simultaneously correspondingly rotated but is also axially traversed due to the co-operation of the screw thread on its keying boss 3% engaging with the internal screw thread 31 of the mounting bush 9.

in order to limit the extent to which the operating spindle 23 can be rotated, i. e. so as to limit it in this particular case to three complete revolutions in either direction, the keying boss 30 is furnished with a stoppin 35 which may be formed of stainless steel and which is fixed in the keying boss 30 so as to intersect the axis of the fork 25 and to lie at right-angles to the flats provided on the keying boss, the ends of the pin 35 projecting beyond these flats and each extending into one of a pair of identical longitudinally arranged closedended slots 36 furnished in the flanks of the head 34 on either side of the bifurcation of the latter. Thus, when the operating spindle 23 has been turned through three complete revolutions in one rotational direction, the ends of the stop-pin 35 will abut one pair of ends of the slots 3% in the operating spindle head 34, whilst, when the spindle 23 has been rotated through three complete revolutions in the opposite direction, the ends of the pin 35 will engage the other ends of the two slots so that no manual effort applied to the operating spindle 23 in any ordinary circumstances can cause any damage to the fork 25, the brush carrier 5, the resistance element 2 or collector element 3. In other Words, the manual drive of the potentiometer is divorced from the brush and the extent to which this can be operated is limited by means independent of the operating knob or the like of the potentiometer.

In a modified stop arrangement, illustrated in Figure 4, instead of the keying boss 30 itself having an external screw thread, the ends of the stop-pin 35 are arranged to project through the slots 36 in the flanks of the bifurcated head 34 of the operating spindle 23 and have a screw thread which engages in the internal screw thread 31 of the mounting bush 9.

The mounting bush 3 is conveniently of hexagonal form at the end thereof mounted without the open end of the body 10, the opening 14 in this end of the body being designed to receive the bush 9 as a close fit therein and in a non-rotatable manner. The axial location of the mounting bush in the body is conveniently provided for by forming the bush with a perimetrical flange 37 which engages in an annular recess moulded in the two halves of the body 10 around the opening 14.

In order to seal the operating spindle 23 within the mounting bush extension 15, the portion of the spindle lying within the extension is preferably provided with two or more annular grooves 38 in which are arranged compressible sealing rings 39 such as O-rings.

The length of spring wire forming the brush 6 conveniently comprises a semi-circular intermediate portion which is moulded into the brush carrier so as to lie symmetrically around the axis of the carrier and with the end portions of this semicircular portion of the brush lying on the diameter of the carrier, these end portions of the semi-circular intermediate part of the brush then bending out of the plane of the latter portion of the wire at right-angles thereto so as to extend transversely to the end planes of the carrier and eventually to project beyond one end of the brush carrier whereafter the two end portions of the wire are bent again through a rightangle so as to extend outwardly from the brush carrier 5 in opposite directions. These outwardly directed portions of the wire are bent to a semi-circular form and lie in planes at right-angles to the axis of the potentiometer, the extremities of these portions being bent at rightangles to these planes so as to form the parts 19 and 20 which lie parallel to the axis of the potentiometer and respectively wipe over the inner periphery of the resistance element 2 and the collector element 3.

The collector element 3 is made of a suitable electrically conducting material which is conveniently nickel silver wire wound to helical spring form and terminated at its ends with bound and soldered tinned copper wire. An insulated lead 40 extends from one end of the collector element through a duct in the wall of the body to the exterior thereof for connection to a terminal.

The resistance element 2 may be of any suitable form and may, for example, be a metallised glass fibre. However, it is conveniently formed of a resistance wire, advantageously oxidized Nichrome wire, wound upon an insulated wire former 41 on which the resistance Wire is secured against displacement by means of thermo-setting resin. The former 41 may conveniently comprise a resincoated copper wire and the coating should be smooth and should not crack or flake during the winding of the former to the three-turn helical form required for the potentiometer. The need for the surface of the insulated layer of the former to be smooth and even, is to ensure that the operative surface of the resistance wire wound thereon is also smooth and even to ensure satisfactory operation and to facilitate cleaning of the resistance element.

In a potentiometer having an external diameter in the region of 1 /2 inches and an overall length, including the operating spindle of 2 /2 inches, the inside diameter of the helix to which the former 41 approximately 1 inch.

The resistance wire is locked in position on the former 41 by the said thermo-setting resin coating but has the resin removed over the whole of the circumference of the resistance wire at the ends of the resistance element and the latter is terminated by binding and soldering tinned copper wire on the cleaned circumference of the end coils of the resistance wire. From the two ends of the resistance element are taken leads 42, encased in insulating sleeving 43, passed through appropriate ducts 44 moulded in the wall of the body 10 of the potentiometer, the leads 42, and any additional tappings from the resistance element, being carried to the exterior of the said two-piece body for connection to appropriate terminals.

The inwardly facing peripheral surface of the resistance element 2 has the thermo-setting resin used for aifixing the resistance wire to the former 41 removed therefrom along a continuous helical path so as to bare the successive coils of the resistance wire for engagement in a good electrically conducting manner with the part 19 of the brush 6 adapted to wipe over the exposed portions of the coils of the said resistance wire.

If desired, the potentiometer may be encased in an outer casing which may comprise a cylindrical metal can adapted completely to receive the body of the potentiometer coaxially therein with the mounting bush and the operating spindle of the potentiometer projecting through the closed end of the can whilst the other end of the latter is closed by a cover or lid soldered or otherwise sealed onto or into the open end of the can, this lid conveniently carrying sealed-in terminals to which the various leads of the potentiometer are connected in ternally of the can. Preferably the said casing Will have in one of its walls at least one external depression providing an inward protrusion keying into a recess formed in the potentiometer body, the relative rotation of the casing about the axis of the said body thus being prevented.

A sealing ring 45 may be provided between the outer surface of the mounting bush and the adjacent end of the casing, this sealing ring conveniently being located in an annular recess in the mounting bush around the said bush extension. Moreover, the casing may conveniently be secured upon the potentiometer body by an internally screw threaded ring screwing onto the exterior of the mounting bush extension which is appropriately screw threaded, as shown, for this purpose and this screw threaded locking ring may itself be furnished with an annular recess to receive a sealing ring for engagement with the back of a panel or other structure to which the potentiometer is to be fixed by means of a further securing ring screwed on to the said mounting bush extension.

The said ducts for receiving the leads to and from the resistance element and the collector element are conveniently formed at the meeting faces of the two halves of the body of the potentiometer, partially in one half and partially in the other so as to facilitate the arrangement of these leads in these ducts.

It will readily be understood that a potentiometer constructed as above described is of very simple but efficient and robust construction and is one which will withstand comparatively rough manual action of the operating spindle without damage to the inner parts of the potentiometer. Moreover, it will readily be appreciated that the potentiometer is capable of rapid assembly or disassembly which enables speedy production of the potentiometer or rapid servicing of the same. The potentiometer is also remarkably compact.

I claim:

1. An electrical instrument comprising: mounting bush secured to said body for the is wound, would be a body; a purpose of mounting the instrument to an appropriate support and having a screw-threaded part; an electrical member which is carried by said body for rotation relatively thereto and the rotation of which causes variations of an electrical property of the instrument; an operating spindle mounted in the instrument immovable axially of the said body, but rotatably relatively thereto; such operating spindle having a diametrical bifurcation in one end thereof; an intermediate driving element drivingly connected to the said electrical member and positioned coaxially of the operating spindle at the end thereof having said bifurcation; a keying boss furnished diametrically across that end of the driving element which is nearer to the operating spindle such keying boss engaging as a free sliding fit within the bifurcation of the latter and having a screwthreaded part engaged with the said screw-threaded part of the mounting bush so that rotation of the operating spindle causes rotation and relative axial movement of the driving element and stop means to limit the axial movement of the driving element.

2. An electrical instrument comprising: a body; an electrical member which is carried by said body for rotation relatively thereto and the rotation of which causes variations of an electrical property of the instrument; a mounting bush secured in said body and having a screwthreaded bore therethrough; a reduced tubular extension on said mounting bush projecting outwardly of said body; an operating spindle mounted rotatably, but non-axially movably, within the said mounting bush extension, an enlarged head at one end of said operating spindle and positioned within the said mounting bush, such head having a diametrical bifurcation therein; an intermediate driving element positioned coaxially of the operating spindle and having its end remote from the operating spindle drivingly connected to the said electrical member; a keying boss furnished diametrically across that end of the driving element nearer to the operating spindle, such keying boss engaging as a free sliding fit within the bifurcation of the head of said operating spindle and being screw-threadedly engaged with the bore of the mounting bush so that rotation of the operating spindle causes simultaneous rotation and relative axial movement of the intermediate driving element; and a stop pin on the said keying boss engaging in at least one slot in the head of the operating spindle the ends of such slot being closed acting to limit the axial movement of the driving element relative to the operating spindle and thereby to limit the rotation of the latter.

3. An electrical instrument comprising: a body; an electrical member which is carried by said body for rotation relatively thereto and rotation of which causes variations of an electrical property of the instrument; an operating spindle mounted in the instrument immovable axially of the said body, but rotatable relatively thereto; intermediate driving element drivingly connected to said electrical member and positioned coaxially of the operating spindle; interfitting parts on the driving element and operating spindle which cooperate so that rotation of the latter causes rotation of the driving element; a stop element on the said intermediate driving element and passing through a slot in the operating spindle, such stop element being screw-threadedly engaged with a correspondingly screw-threaded part of the said body so that, as the driving element rotates, it also moves axially, the extent of axial movement being limited by the engagement of the said stop element in the said slot.

4. A helical wire wound potentiometer comprising: a hollow body formed of electrically insulating material; a hclically coiled resistance element located within said body; a lead screw coaxially mounted within said resistance element and having a thread of the same pitch as the resistance element; a nut rotatably mounted on said lead screw; a brush on said brush carrier engaging the resistance element; an intermediate driving element mounted Within said body and coaxially of the said lead screw, said driving element having a forked portion which O it) embraces the said nut so that the latter is free to move axially of the resistance element whilst simultaneously being rotated about said lead screw; an operating spindle mounted coaxially of the said driving element and having a portion projecting outwardly of said body and a diametrical bifurcation in its inner end; a keying boss furnished diametrically across that end of the said driving element which is nearer to the operating spindle, such keying boss engaging as a free sliding fit within the bifurcation of the operating spindle and having a screw threaded part threadedly engaged with a relatively stationary screw-threaded part of the instrument; and cooperating stop means on the operating spindle and driving element which act to limit axial movement of the driving element.

5. The helical wire wound potentiometer specified in claim 4-, wherein the relatively stationary screw-threaded part of the instrument with which the screw-threaded part of the said keying boss engages is a mounting bush for the instrument, such mounting bush also providing a bearing for the said operating spindle.

6. A helical wire wound potentiometer comprising: a hollow body of electrically insulating material; a helically coiled resistance element located within said body; a helically coiled collector element located within said body coaxially with, but axially offset from, the resistance element; a lead screw coaxially mounted within said resistance element and collector element; a brush carrier of insulating material mounted rotatably on said lead screw; a brush mounted on said brush carrier and having portions engaging the resistance element and collector element; a mounting bush secured in said body and having a screw threaded bore; a reduced tubular extension on said mounting bush projecting outwardly of said body; an operating spindle mounted rotatably, but non-axially movably, within the said mounting bush extension; an enlarged head at one end of said operating spindle and positioned within the said mounting bush, such head having a diametrical bifurcation therein; an intermediate driving element positioned coaxially of the operating spindle and having its end remote from the operating spindle of forked form and embracing the said brush carrier; a keying boss furnished diametrically across that end of the driving element nearer to the operating spindle, such keying boss engaging as a free sliding fit within the bifurcation of the head of said operating spindle and being screw-threadedly engaged with the bore of the mounting bush so that rotation of the operating spindle causes simultaneous rotation and relative axial movement of the intermediate driving element; and a stop pin on the said keying boss engaging in at least one slot in the head of the operating spindle, the ends of such slot being closed and acting to limit the axial movement of the driving element relative to the operating spindle and thereby to limit the rotation of the latter.

7. An electrical instrument comprising: a body; an electrical member which is carried by said body for rotation relatively thereto and rotation of which causes variations of an electrical property of the instrument; an operating spindle mounted in the instrument immovable axially of the said body, but rotatable relatively thereto; an intermediate driving element drivingly connected to said electrical member and positioned coaxially of the operating spindle, such intermediate driving element having a screw-threaded part engaging with a complementary screw-threaded part fixed relatively to said body so that rotation of said element relatively to the body must be accompanied by simultaneous axial movement of the ele ment relatively to the body; interfitting parts on the intermediate driving element and operating spindle which cooperate so that rotation of the latter causes rotation of the intermediate driving element, but which permit the intermediate driving element to move axially of the operating spindle; and cooperating stop means on the operating spindle and intermediate driving element acting to in the instrument immovable axially of the said body, but 10 rotatably relatively thereto; an intermediate driving element drivingly connected to said electrical member and positioned coaxially of the operating spindle, such element having a screw-threaded part engaging with said screwthreaded part of said mounting bush so that rotation of 15 said intermediate driving element produces simultaneous axial movement of the element relatively to said body; complementary interfitting tongue and groove means on the intermediate driving element and operating spindle acting to ensure that rotation of the operating spindle causes rotation of said intermediate driving element, whilst allowing the latter to move axially of the operating spindle; and co-operating stop means on the intermediate driving element and a relatively non-axially movable part of the instrument acting to limit the axial movement of the intermediate driving element.

References Cited in the file of this patent UNITED STATES PATENTS 1,606,153 Douglas Nov. 9, 1926 2,342,830 Bate Feb. 29, 1944 2,371,159 Erb Mar. 13, 1945 2,495,321 Gibbs et al. Jan. 24, 1950 2,665,355 Van Alen et a1. Jan. 5, 1954 

